The invention relates to a device for the electromagnetic transmission in an interference-laden environment of a sporting event, in particular an event entailing the start of a race involving several racers, said event being defined by the time t.sub.e at which it occurs.
When it is necessary to transmit events occurring at decentralized locations to a central management point, during races at sporting events for example, there are two generally known means used: transmission by cable or transmission by electromagnetic means.
An example of the first case is set forth in U.S. Pat. No. 4,156,870, in which the peripheral units are connected through an electronic module to a central control, operating and data processing unit through a single twin-strand cable. In the event there is a great distance separating the events to be measured from the central control point--ski races or practice races, for example--it may be advantageous to transmit the events in question by electromagnetic means (microwaves, light waves or infrared beams). This precludes the need to string long lines which, moreover, run the risk of accidentally being severed. Even on a course covering a relatively confined space, such a system may be advantageous because it eliminates considerable set-up and preparation time.
The radio link, known in and of itself, has disadvantages as well as advantages. In particular, it is affected by interference which may be of sufficient proportions to prevent the guaranteed transmission of the events. This refers in particular to atmospheric interference or interference from nearby transmitting equipment. These kinds of interference can prevent the transmission of a one-time event. It is readily understandable that if said event is characterized, for example, by the arrival time of a racer, as in sports timing, the times posted will be irrevocably lost.
To overcome this difficulty, it has been proposed to repeat the transmission of the message several times and for sufficiently long a period of time for its reception to be assured. In the event the message consists essentially of the time at which the event occurred, it will be repeated five, ten or twenty times, with the value always the same. If the word time is taken here to mean the time indicated by a master clock--which may be calibrated on the time of day provided by the standards service's clock, for example--at the loction where the event is taking place, it will be necessary to have an extremely precise time base if the aim is to measure the starting times of competitors in a race. Indeed, the time required to run a particular distance is determined by the difference existing between the starting time and the finishing time. It follows that each starting and finishing point will have to be equipped with extremely accurate clocks, synchronized as required.
An example will serve to clarify ideas. The lapse of time between the departure of the first racer and the arrival of the last racer in a given competition is, let us suppose, two hours. During these two hours, it is desired that the time shown by the clock at the starting point and the time indicated by the clock at the finish line diverge by no more than one thousandth of a second. Under these conditions, the precision required of each clock will be 0.001.times.24/2=0.012 second per day. Such a degree of precision can be attained only by a sophisticated and expensive device which, moreover, will have to be stabilized at a temperature range of -20 to +60 degrees Celsius.
Certificate SU-A-183,413 proposes to improve the sureness of radio transmission of the instant at which a charge used in seismic research is fired off. The system is based on the transmission of signals triggered by the charge and coded in series, with the coding changing each second. Thus, when one of the signals is received correctly, the instant of firing can be evaluated.
Apart from the fact that the said document does not mention how the use of such a system might be applied to sports racing chronometry, it also does not consider or suggest that the coded signals are arranged in such a way as to provide, in addition to the indications which situate them in time vis-a-vis the event, indications whereby it is possible to identify each racer taking part in the competition. Furthermore, in the last paragraph of Column 1 of the said document, it is stipulated that the generator 1 shown in FIG. 1 is quartz stabilized, indicating that in this system it is the transmitter which contains the time base and not the receiver (see also the last paragraph of Column 3), quite contrary to what will be described in the present invention in which the transmitter has only a relatively imprecise oscillator and it is the receiver that contains a precision time base.
While the present invention is also based on the idea of the repeated transmission of events, it proposes to overcome the above-mentioned drawbacks.